Conventional turbo props are typically used at lower cruise speeds where they provide good performance and high efficiency. At higher cruise speeds, it is typically required to use a turbo fan to produce the relatively high thrust required. A scaled up version of a conventional turbo prop engine suitable for powering an intermediate-sized transport aircraft at the required cruise speeds and altitude would require excessivley large propeller diameters and would require the capability of generating higher shaft horsepower than is conventionally possible. While these high bypass ratio engines are efficient, the rotational speed of the large diameter propeller is a limiting factor in the use of such engines. It is generally required to keep the helical velocity of the propeller tip below supersonic speeds. Propeller tips operating at supersonic speeds generate a significant amount of undesirable noise and results in a loss of aerodynamic efficiency.
The turbo machinery for aircraft having such fans which have a bypass ratio greater than about eight are generally arranged to use a speed change gearbox in order to reduce the speed of the fan rotor relative to the speed of the turbine. The speed change gearbox provides the method for a more optimum fan blade speed for higher efficiency along with a high speed, smaller diameter turbine drive shaft and a high speed turbine with fewer stages. However, a gear box and associated accessories result in a significant increase in engine complexity and weight.